Retinopathy of prematurity (ROP) causes significant visual compromise or loss in >2000 infants/yr in USA, even with therapy. Case control studies, anecdotal reports, and animal studies report successful treatment of moderately severe ROP with supplemental oxygen. This suggests that retinal hypoxia may be a critical factor associated with ROP. This study will test the hypothesis that supplemental therapeutic oxygen for presthreshold ROP will reduce by at least 1/3 the number of infants with one or both eyes progressing to threshold ROP.